1. Field
The disclosure of the present patent application relates to photovoltaics, and particularly to a porous silicon nanowire photovoltaic cell using an array of porous silicon nanowires as a p-n junction, as well as an additional up-conversion layer.
2. Description of the Related Art
Photovoltaic devices typically employ a planar thin-film structure in which a negatively doped (n-type) material is stacked on top of a positively doped (p-type) material, or a positively doped (p-type) material is stacked on top of a negatively doped (n-type) material. In these planar photovoltaic devices, the light absorbing layer needs to be thick enough to effectively absorb impinging photons with energies larger than the bandgap energy of the light absorbing material. However, when the light absorbing layer in a planar structure is made thicker, this compromises the effective collection of the photo-generated carriers as the thickness of the light absorbing layer may be larger than the diffusion length of the minority carriers. Thus, the design of typical planar photovoltaic devices leads to a compromise between the thickness of the light absorbing layer for efficient light absorption and the effectiveness of carrier collection, thereby imposing limits on the efficiencies of these devices. As an example, a typical thin-film GaAs solar cell requires a light absorbing layer several microns thick to effectively absorb photons with energies higher than its bandgap energy, but as the diffusion length of the minority carriers is typically only a few hundred nanometers, many of the photo-generated carriers cannot be collected.
Rather than using planar p-n junctions in photovoltaic devices, radial p-n junctions are presently under investigation. In these structures, a long central n-type core extends out of a substrate and a p-type shell is wrapped around the core. In alternative configurations, the core is of a p-type material, while the shell is formed of an n-type material. Examples of such materials under investigation include nanowires formed from a GaAs core surrounded by an AlGaAs shell, as well as ZnO nanowires used in organic dye-based photovoltaic cells. Using such structures, one of the two photo-generated carrier types is collected in the shell orthogonally to the light absorption along the length of the core. Unlike in planar p-n junctions, increasing the length of the core to improve light absorption does not increase the distance the carriers need to travel before being collected, and therefore does not lead to the trade-off in light absorption and carrier collection found in typical planar devices.
Recent developments in the fabrication of nanowires extending out of substrates have made it possible to manufacture radial p-n junction photovoltaic devices. However, the efficiencies that have been achieved with these radial p-n junctions have been substantially less than corresponding planar devices, typically achieving solar cell efficiencies of less than 10%. It would be desirable to be able to fabricate a nanowire-based photovoltaic cell with efficiencies similar to, or surpassing, those of planar silicon photovoltaic cells.
Thus, a porous silicon nanowire photovoltaic cell solving the aforementioned problems is desired.